In conventional switching power source arrangement, it has been common to provide such control circuit with an analogue circuit. In this instance, the output voltage and the output current are detected in terms of analogue signals which are processed in the form of analogue values, the results of the processing being used to produce control signals for controlling the switching operation.
In general, in switching power sources, operating modes of power converter circuit may be changed in accordance with changes in input voltage or load current. In this instance, it is required that a plurality of control algorithms be provided for the respective ones of the control modes, which are appropriately selected to accomplish the most appropriate control. It should however be noted that the conventional switching power source including such analogue control circuit is designed to perform a signal processing in the control circuit mostly based on a hardware circuit structure so that control circuits or hardware structures have to be provided in advance for the respective ones of the control algorithms so that they can be selectively used in accordance with the operating modes in order to accomplish the most appropriate control through a selection of a control algorithms in accordance with a change in the operating mode in the power converter circuit. This will mean that a plurality of hardware control circuits have to be provided. The structure is therefore complicated in circuit construction so that it is practically unsatisfactory.
In a conventional switching power source having an LC filter and diode for eliminating ripple, there are produced two different operating modes, one being a continuous mode and the other a discontinuous mode, which are produced depending on the value of the load current. It is common in a conventional switching power source utilizing an analogue control circuit to use, even under an operating condition wherein the inductor current is discontinuous, methods or sequences of signal processing which are designed with control models under conditions wherein the inductor current is continuous. This is because the fact that it is difficult or make the structure complicated and uneconomical to change the methods or the sequences of signal processing depending on the load condition. Therefore, the control models are inappropriate under an operating condition where the inductor current is discontinuous so that unsatisfactory conditions are produced in respect of stabilities in the output voltage and the control systems.
A resonance type power source has been developed primarily for decreasing the switching loss in the switching power source arrangement. However, this technology is still in the process of development so that there is no available unified circuit system. Thus, there is no unified control algorithms in this type of power source for the respective circuit systems and different control circuits are provided for different circuit systems. Thus, there are serious problems in an effort to provide a standardized control circuit comprising a control IC.
A non-resonance type circuit system generally has a plurality of operating modes. Therefore, in order to provide a stabilizing control through an analogue circuit system, it is required to provide a plurality of control circuits for carrying out control algorithms for the respective ones of the control modes and selectively use one of these control circuits in accordance with the op-operating mode. However, in actual practice, this solution further involves a problem in that a complicated control circuit is required.
In a switching power source for a special use such as a battery charger, it is required to provide a constant current characteristic for providing a substantially constant output current and a constant power characteristic for providing a substantially constant output power as well as a constant voltage characteristic for providing a substantially constant output voltage. In this instance, it is also required to provide a plurality of control algorithms for the respective ones of the operating modes since the respective characteristics require different operating modes. It will therefore be noted that in switching power source arrangements having control circuits of analogue type it is required to provide a plurality of control circuits for respective ones of the aforementioned control algorithms and selectively use one of the control circuits in accordance with the operating mode. However, in actual practice, this arrangement has a problem in that the circuit is made complicated as in the case of a resonance type power source.
In trying to perform a control based on the input voltage, the output voltage and the output current in a conventional switching power source having an analogue control circuit, it is required to provide detecting circuits for obtaining information on actual values of such input voltage, output voltage and output current. Particularly, in a power source arrangement having an insulation between the input and the output, it is required to provide insulations in certain portions of signal transmitting lines which are provided for taking the information into the control circuit. Thus, the circuit arrangements become complicated and the number of components in the circuit will be increased.
As describe, in a conventional switching power source having a control circuit of analogue type, it is required to provide different hardware such as different circuits or circuit components in order to enable such selective use or change of the control algorithm. This will mean that hardware standardization will be seriously disturbed and as a result it necessary to produce a variety of products in a small quantity, so that there will be problems of reduction in the design efficiency and productivity.
Further, as a matter of general consideration, it is necessary to take into account the relationship between the switching power source and the load device. It is common that the output of the switching power source be connected through a load line to a load device. In this instance, it is ideal that the voltage supplied by the switching power source be controlled in such a manner that the voltage across the load terminals at the opposite ends of the load is maintained constant. However, in many switching power sources, voltage detecting circuits are provided in the switching power source so that control is made on the output voltage at the output portion of the switching power source to maintain the output voltage constant. This type of control has a problem that the voltage across the load terminals becomes lower than desired due to a voltage drop at the load line.
There may be several solutions for the problem. However, in the system wherein the voltage at the output portion of the switching power source arrangement is controlled to be constant, none of them will be effective to provide a satisfactory control under a possible fluctuation of load. There is apparently a limit in solving such problems if the system utilizes a switching control by means of an analogue circuit.
The Japanese Laid-Open Patent Publication No. Hei 2-74152 discloses a switching power source having a digital control device adopted for controlling the switching circuit. The switching power source disclosed in this patent publication includes a switching circuit for switching an input electric power and a smoothing circuit for smoothing the output of the switching circuit to provide an output, a digital controller being provided for controlling the switching circuit. The digital controller produces control pulse signals of a predetermined duty factor in accordance with the value of the output from the switching power source, the control pulse signals being applied to the switching circuit to control the operation of the switching circuit.
The patent publication teaches for a switching power source including a digital control means of this type a technique for providing an appropriate control in response to fluctuations in the input voltage to the switching power source which may be produced when there are fluctuations in the output of a power source which is provided for supplying an input to the switching power source. More specifically, the patent publication recommends a structure wherein a plurality of processing formulas are provided and a desired one of the formulas is selected in accordance with a detected value of the input voltage to the switching power source. In this instance, the input voltage is detected by input voltage detecting means.